A superconducting material exhibits its superconductivity at and below its critical temperature (Tc) and oxide superconducting materials having a relatively high critical temperature (Tc) are expected to find applications at the boiling point, or 77 K., of liquid nitrogen. Two methods are generally used for cooling superconducting materials. One involves the use of a freezer and the other utilizes liquid helium or nitrogen as a medium of freezing. The latter is normally recommended for cooling superconducting coils and bulk superconducting materials from the viewpoints of rapid conduct ion of heat, enhanced thermal conductivity and even distribution of heat. Liquidized helium is often used under reduced pressure at temperature below 2.19 K. to keep it in a superfluid state. In view of the above described facts and other considerations, the temperature at which a bulk oxide superconducting material is used is preferably 2.19 K., 4.2 K. or 77 K.
A superconducting material normally needs to be cooled considerably below its critical temperature in order to ensure its desired properties in a stable manner under high electric current density condition. While the use of liquid helium (2.19 K., 4.2 K.) as cooling medium provides an advantage of increased critical electric current density when compared with the use of liquid nitrogen, it is accompanied by the disadvantage of high cost and difficulty of handling. As for the use of liquid nitrogen (77 K.), on the other hand, there has been a report that a QMG material prepared by a quench and melt growth method and cooled by liquid nitrogen (77 K.) exhibited a Jc value of 30,000 A/cm.sup.2 in a magnetic field of 1 T ("New Superconducting Materials Forum News"; No. 10, p. 15) and another report says that a Jc value as high as 4,000 A/cm.sup.2 has been achieved by using a Bi-type silver-sheathed wire, suggesting that such superconducting materials may find practical applications in near future. It is widely recognized, however, that a new or improved cooling method has to be proposed that can cool oxide superconducting materials below 77 K. in a stable manner, using easily handled liquid nitrogen as cooling medium so that the superconducting properties of such materials may be fully exploited.
It has also been reported that a maximum magnetic flux density of 1.35 X 10.sup.-2 T was achieved at 77 K. by a bulk magnet made of a QMG material but it was accompanied by creep phenomenon in the magnetic flux of the QMG material that gradually attenuates the density of magnetic flux with time. Therefore, there is also urgent need for a remedy for such creep.